Neurodiagnostics

Question 1

Describe the functional components of the motor unit.

Answer 1

Question 1

What is the broad classification of neuromuscular dieases?

Answer 1

Neuropathies, junctionopathies, myopathies, neuromyopathies

Question 2

What is the gold standard test for myasthenia gravis?

Answer 2

Acetylcholine receptor antibody test

Question 3

What are 6 tests that can be performed as part of a minimum database for neuromuscular disease?

Answer 3

Creatinine kinase - sensitive indicator of skeletal muscle damage.

Serum electrolytes - alterations can result in muscle weakness.

Blood or plasma lactate or pyruvate - associated with inborn metabolic errors.

Urine myoglobin - may be elevated following significant muscle injury.

Thyroid screening - hypoT may cause lower motor neuron weakness.

ACh receptor antibody - myasthenia gravis.

Question 4

How can masticatory myositis be definitively diagnosed?

Answer 4

Measurement of serum antibodies directed against 2M myofiber

Question 5

What two groups of tests are specific for diagnosis of neuromuscular disease?

Answer 5

Electrodiagnostic testing, muscle/nerve biopsy

Question 6

What is the most common type of electrode used in electromyography?

Answer 6

Concentric needle electrode

Question 7

What are the four types of spontaneous electrical activity observed in normal muscle during electromyography?

Answer 7

Insertional activity - triggered by needle insertion.

Miniature end plate potentials - when the needle is in close proximity to a neuromuscular junction.

End plate spikes - when a single normal myofiber depolarizes completely.

Motor unit action potentials - recorded from a normal muscle that is not completely at rest.

Question 8

What are the two types of electrodiagnostic testing most frequently used in veterinary medicine?

Answer 8

Electromyography - detection of electrical activity recorded from a patient’s muscles.

Nerve conduction studies - measurement of nerve conduction velocities of motor and sensory nerves.

Question 9

What are some types of abnormal spontaneous activity that may be observed during electromyography?

Answer 9

Fibrillation potentials, positive sharp waves, complex repetitive discharges, myotonic potentials.

Question 10

During motor nerve conduction velocity testing what is the main waveform observed?

Answer 10

M-waves: orthodromic propagation of action potentials along a nerve, acetycholine release at the NMJ, myofiber depolarization.

Question 11

What 2 findings on a motor nerve conduction study might suggest demyelination?

Answer 11

Polyphasia of M-waves, extreme slowing of conduction velocity

Question 12

Can velocity testing also be performed on sensory nerves?

Answer 12

Yes, in a similar fashion as motor nerve testing except the nerve is directly tested rather than the muscle that it innervates (i.e. stimulation of a sensory nerve and recording of the action potential along the course of that nerve).

Question 13

What are somatosensory evoked (or cord dorsum) potentials used for?

Answer 13

To evaluate the peripheral and central nervous systems following sensory nerve stimulation

Question 14

What are the different types of ‘waves’ that are typically observed with somatosensory evoked potential testing?

Answer 14

M-waves and late responses (F-waves and H-waves). Late responses indirectly evaluate nerve root function.

F-waves: antidromic propagation of action potentials along the motor nerve, to the ventral gray column of the spinal cord, and then back again.

H-waves: reflex involving afferent sensory fibers to the dorsal horn, and then efferent motor neurons in the ventral horn.

Question 15

What are two electrophysiologic techniques that are useful in assessing the neuromuscular junction?

Answer 15

Repetitive nerve stimulation: repeated stimulation of a motor or mixed nerve while recording M-waves from the muscle innervated by the nerve.

Stimulated single fiber electromyography: assesses neuromuscular ‘jitter’ following repeated stimulation with a concentric needle electrode. Increased jitter is consistent with subtle pathologic changes at the NMJ (such as with myasthenia gravis)

Question 16

What finding would be expected on repetitive nerve stimulation in a patient with myasthenia gravis?

Answer 16

Decremental decrease in response (>10%)

Question 17

What conditions can muscle and nerve biopsies broadly differentiate?

Answer 17

Neuropathies, junctionopathies, myopathies

Question 18

What components of the motor unit does muscle biopsy allow evaluation of?

Answer 18

Motor unit, intrafasicular nerve branches, NMJ, myofibers, supportive connective tissue, vascular tissues.

Question 19

Name 5 selection criteria for a muscle biopsy (there are 8 total)?

Answer 19

1. Should be affected by the disease process.
2. Should be easily identified, surgical approach should be minimal, fibers should be orientated in a single direction.
3. Procedure should be low morbidity
4. Samples should be collected from muscles with which there are previous interpretive experience
5. Harvested from a site remote to tendon insertions and aponeuroses
6. Free of artifacts
7. Specialized samples if required
8. Discussion with the lab prior to sample collection to ensure maximum diagnostic info

Question 20

What are the two main methods of muscle biopsy collection (with some advantages and disadvantages for each)

Answer 20

Percutaneous needle: quick, inexpensive, can be performed under local. Small samples size, and inability to keep fibers stretched when fixing samples.

Open muscle biopsy: opposite of above

Question 21

How should a muscle biopsy be stored after collection?

Answer 21

Frozen (or placed in fixative if required for electron microscopy)

Question 22

What are the criteria for selection of a nerve biopsy site?

Answer 22

1. The nerve should be affected by the disease process, as evidenced by abnormal electrophysiologic investigation, neurologic abnormalities).
2. Low morbidity, established data available, innervates a muscle that is routinely biopsied.

Question 23

Which nerve is commonly biopsied in instances of generalized neuromuscular disease?

Answer 23

The common fibular nerve (mixed nerve containing motor, sensory and autonomic nerve fibers).
Other mixed nerves that can be samples include the tibial and ulnar.

Question 24

If both dorsal and ventral nerve roots are affected, biopsy of which nerve root is preferred?

Answer 24

Dorsal as normally results in minimal neurologic deficits

Question 25

Describe a diagnostic plan for the work-up of neuromuscular diseases

Answer 25

Question 26

In a study by Troupel 2021 in JVIM, what electrodiagnostic finding indicated a higher likelihood of clinical improvement in dogs/cats with traumatic brachial plexus injury?

Answer 26

Increased amplitude of the radial nerve muscle action potential was associated with increased clinical improvement (specifically a amplitude threshold >5mV).

Question 27

In a study by Dell’Apa in 2024 in JVIM, what electrodiagnostic finding indicated a higher likelihood of clinical improvement in dogs with sciatic nerve injury? What clinical examination finding was associated with a positive outcome?

Answer 27

An increase in the compound muscle action potential >1.45mV was associated with an increased likelihood of improvement.

Clinical ability to flex and extend the hock was also associated with a greater chance of recovery.